1. Field of the Invention
The present invention relates to a positive electrode material for a lithium secondary battery, and to a manufacturing method thereof.
The present application claims priority to Japanese Patent Application No. 2015-157774 filed on Aug. 7, 2015, the entire contents of which are hereby incorporated by reference.
2. Description of the Related Art
Lithium secondary batteries (lithium ion secondary batteries) have recently found use as the so-called portable power sources for, for example, personal computers and portable terminals, and as drive power sources for vehicles. In particular, lightweight lithium secondary batteries (lithium ion secondary batteries), which make it possible to obtain a high energy density, have been advantageously used as high-output drive power sources for vehicles such as electric vehicles and hybrid vehicles.
When a lithium ion secondary battery is charged, lithium ions are released (desorbed) from the interior of a positive electrode active material, and when the battery is discharged, the lithium ions are conversely stored (inserted) in the positive electrode active material. The positive electrode active material is known to expand and contract repeatedly following such storage and release (typically, insertion and desorption) of lithium ions.
The positive electrode active material is typically used in the form of secondary particles formed by aggregation of a large number of fine primary particles. It is well known that due to the expansion and contraction of the positive electrode active material, secondary particles of the positive electrode active material may induce fracture (including cracking) between the primary particles with weak bonding forces. Typically no contact with an electrically conductive material is realized in the locations where the fracture has occurred (fractured portion). Therefore, the electron conductivity decreases and the formation of conduction paths between the positive electrode active material particles is likely to be inhibited. Thus, the internal resistance of the battery can rise and cycle characteristics can degrade.
In order to resolve this problem, Japanese Patent Application Publication No. 2013-191579 discloses a method for suppressing the generation of fracture between the primary particles of a positive electrode active material by using a positive electrode material including a lithium compound on the outer circumferential surface of the positive electrode active material particles.